September 27, 2013

The U.N. Is 95 Percent Certain Humans Cause Climate Change. What About the Other 5 Percent?

Early Friday morning, members of United Nations Intergovernmental Panel on Climate Change released a summary of their fifth mammoth assessment of all the world’s best scholarship on global trends in climate change. The outlook, predictably, is not good. The planet is absolutely warming at a pace that is “unprecedented.” And the report’s authors can say with 95 percent certainty that the prevailing cause of climate change is human activity.

Which is pretty darned certain. But this is a report which makes many of its assertions with an absolute level of confidence; it calls the evidence that the planet is warming “unequivocal.” So what about the other 5 percent?

“Science follows a hierarchy of certainty,” said Gabriel Vecchi, who works for the long-term research arm of the National Oceanic and Atmospheric Administration and was a contributing author to the report. “You’re always going to be more confident in what you’ve seen than what causes what you’ve seen.” Vecchi said that 5 percent represents the odds that in the future, the IPCC will have to revise this painstakingly crafted statement: “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

“But I think 95 percent is a pretty substantial level of confidence,” Vecchia added. “One of them could be that instead of the dominant cause of planet warming—50 percent plus one—we learn over time that there could be another cause. But each one of the ways that you could revise that statement only adds up to 5 percent.”

Dennis Hartmann, a professor of atmospheric sciences at the University of Washington and another report author, explained that the exact figure “95 percent” comes about as a result of some complicated modeling. “Basically it requires observations, an estimate of natural and anthropogenic forcing of climate change. Then you run many models and you find that you cannot reproduce the observed warming unless you have the human forcing,” he wrote in an email. (“Human forcing” is an all-encompassing term that includes the carbon-dioxide emissions produced by human activity.) “You can make an objective case that it is 95 percent certain that you can only get the observed warming of the 20th century [if] you have human forcing.”

The bottom line is, the reason for the groups’ hedging is technical—a product of this report being an assessment of all the most vetted literature on climate change, and not a new study in its own right. Their level of confidence does not reflect a 5 percent chance, for instance, that Wisconsin Rep. Jim Sensenbrenner was right all along, and solar flares really are responsible for warming our planet. And more likely than not, mounting evidence will cause the IPCC in the future to revise that 95 percent upwards.

Molly Redden is a New Republic staff writer. Follow her on Twitter @mtredden.